Electrostatic precipitator



SAMPLING CHAMBER Ju'ly 22, 1958 w. c. HALL ETAL 2,344,214

ELECTROSTATIC PRECIPITATOR Filed July 11, 1955 /e EFFIGI ENGY EOUTPUT S.LNlOd MOHHO ALISNHCI :10 NO] EXHAUST BLOWER MEASUR- INVENTORS EDWARD J. WEST DOMINIC S. TOFFOLO WAYNE C. HALL SOURCE OF CONTAMINATED GAS POWER SUPPLY BY 4?- M i ATTORNEYj' United States Patent() ELECTROSTATIC PRECIPITATOR Wayne C. Hall, Oxon Hill, and Dominic S. Tolfolo, Camp Springs, Md., and Edward J. West, Washington, D. (3., assignors to the United States of America as represented by the Secretary of the Navy Application July 11, 1955, Serial No. 521,425

(Ilaims. (Cl. 183-7) (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates in general to devices for removing small particles in a gas by electrostatic precipitation and in particular to an improved particle charging means.

In recent years the gas contamination problem has become increasingly more acute and various schemes for removing foreign particles from the gas have been devised. In the various attempts to alleviate this problem, it has been found that larger particles, above microns in diameter, may be readily removed from a gas by, for example, centrifugal devices, simple mechanical filters, washing devices, etc. On the other hand, it has been found that smaller particles, particularly those of only a fraction of a micron in diameter, seem to defy almost all cleaning means with the exception of electrostatic precipitation.

Electrostatic precipitation involves the charging of the minute particles in the gas and the subsequent collection of these charged particles under the influence of an electric field. In one type of precipitator, the parallel plate, the charging and collecting of particles is done in two separate stages. In another type, the Cottrell, the charging of the particles and the collection of the particles are effectively combined in one stage. The Cottrell type precipitator is generally recognized as the superior of the two due to its better efliciency and thus greater economy of operation plus its simplicity of design. However, since electrostatic precipitators generally have a maximum efficiency of approximately 75%, heretofore few have been considered as highly eificient gas filtration means. It is believed the inefiiciency in electrostatic precipitators is primarily due to an incomplete charging of the particles in the gas before collection.

It is an object of this invention to provide an improved electrostatic precipitator in which substantially all foreign particles in a flowing gas may be charged.

It is a further object of this invention to provide an improved electrostatic precipitator which includes a fibrous means for charging foreign particles.

It is another object of this invention to provide a means for charging foreign particles in an electrostatic precipitator which does not reduce the voltage breakdown characteristic of the precipitator.

Other objects will become apparent upon a complete understanding of'the invention for which reference is had to the accompanying drawings and description of the invention.

In the drawings:

- Fig. 1 is a pictorial showing of one embodiment of the invention in a Cottrell type precipitator.

Fig. 2 is a pictorial showing of the charging disc in the embodiment in Fig. 1.

Fig. 3 is a pictorial showing of an alternative charging disc in the embodiment of Fig. 1.

cipitator and the exhaust blower 15.

2,844,2l il Patented July 22, 1958 Fig. 4 is a graph showing the relative efliciency of a typical Cottrell precipitator as compared with the improved precipitator of this invention.

Briefly, this invention provides a more eflicient charging of the minute particles in an electrostatic precipitator. In particular, a fibrous material which is sufliciently porous to permit the passage therethrough of the majority of particles to be collected has been transversely disposed within the precipitator. In one embodiment of the invention, by applying a voltage to charge this fibrous material, high field intensities around outlying fibers are developed and an auxiliary charging field is distributed over the entire cross-section of the precipitator. In other embodiments of the invention, no voltage is applied to the fibrous material and, in one instance, the tribe electric or frictional effect of the particles brushing against the fibers inpassing therethrough charge the particles to be collected. In another instance where no voltage is applied to the fibrous material, charged particles transfer a portion of their charge to the fibrous material which charge is in turn picked up by lesser charged particles passing through the fibrous material. It has been found that the device of this invention affords a highly satisfactory electrostatic gas filtration means suitable for use in almost any contaminated gas application.

More particularly, Figure 1 exemplarily shows a typical Cottrell type precipitator comprising an outer shell conductor l0 and a corona wire 11 which is extended therein. The corona wire 11 is electrically connected to a source of high voltage 12 and is centrically supported within the outer shell 10 by flange pieces, indicated at 13, such that a relatively high potential, 25 kv. for example, may be applied between the corona wire and the outside wall of the precipitator. A source of contaminated gas 14 is disposed at the input end of the precipitator and an exhaust blower 15 is connected to the output end to draw the contaminated gas from source 14 through the precipitator. For purposes of evaluating the eifectiveness of the precipitator, a sampling chamber 16 to which suitable density measuring means 17 is attached, has been inserted in the gas flow path between the pre- It is understood that said sampling chamber 16 and its accompanying density measuring means 17 are both non-essential to the invention.

It should be noted at this point, that in the Cottrell type precipitator, the outer shell acts as the charged particle collector. In a common installation, means, not shown in the drawing, may be provided for periodically removing masses of particles adhering to the shell. For example, means for mechanically vibrating the shell and a sieve for collecting the disadhered masses of particles may be provided.

In accordance with the basic teaching of this invention Fig. 1 shows fibrous material transversely disposed within the precipitator. In the especial embodiment of Fig. l the fibrous material which has substantially the disc configuration shown in Figure 2, is in direct contact with both the outer shell 10 and the corona wire 11 which runs through its center. Thus it may be seen that the voltage applied to the corona wire also acts as a charging means for the fibrous material in this embodiment.

In view of its simplicity, in the Cottrell type precipitator, the above described means for charging the fibrous material is preferred. However, it is to be understood that, where it is desirable to charge the fibrous material, other means of charging may be used in this invention. For example, as shown in Figure 3, the fibrous material may be sandwiched between two metal grids which are indicated at 19, and an external source of voltage, not shown, may be applied across these two grids to charge 'more specifically the spacing between the fibers.

3 the material. In the embodiment of Figure 3, which is especially adaptable to the Cottrell type 'precipitator shown in Figure l, the metal grids and the fibrous disc are cored to avoid contact with the corona wire 11 and the entire disc assembly is separated from the outer shell to avoid contact therewith by insulating ring '20.

The prime purpose of the generally describedffi'brous material is to more fully charge the 'forei'gn particles in the gas, andv in the above described embodiment of this invention it Will be seen thata varietyf o f electrically chargeable fibrous materials, such "as those under the trade-names Fiberglas, Glass Wool, Curleywool etc., may be used. The principal criteria for selecting a particular fibrous material is the size of the fiber, It will be seen that to develop a high voltage field between fibers for charging 'thepart'icles in the gas the spacing must be as small as possible. However, since it is intended that the majority of the-particles to be electrostatically collected pass throughthe fibrous materiaLit is essential that the spacing between fibers be at least equivalent to the size of the particle to be collected. Another important criteria for the selection of the fibrous material is the pressure drop across the material which is caused by its physical impedance, or as it is cornmonly known, its air resistance. In high velocity applications, a third criteria, the mechanical strength of the material, must also be considered. It follows that the material selected and the porosity will necessarily vary in accordance with the particular application. I

It is also within the purview of this invention to employ fibrous discs, such as shown in Fig. 3, in substantially the same arrangement shown in Fig. 1, but without any voltage applied to the fibrous material. In this alternative embodiment, the fibrous disc nearest the input of the precipitator charges the particles by the tribo electric or frictional effect of the particles brushing against the fibers in passing therethrough. The tribo electric phenomenon being peculiar to a variety of composition combinations, the fibrous material utilized in this disc is selected in'accordance with the composition characteristics of the particles to be collected, for example, silkenfibers in'the case of finite glass-like particles, might be employed. For continuous operation of this embodiment, it is'advisable'to dissipate the charge (of opposing polarity to that developed on the particle) which will otherwise build up on the fibrous material. This dissipation may be provided by the simple expediency of grounding the metal grids 19 in Fig. 3, for example.

The centermost fibrous disc inthis alternative embodiment is disposed suificientlywithin the precipitator to intercur particles which have been previously charged by the ordinary charging action of the precipitator and/or by the other fibrous disc. Such charged particles transfer a portion of their charge to this centermost fibrous material, which, of course,'must be chargeable, in passing therethrough and this fibrous material effectively distributes this charge over theentire cross-sectional area of the precipitator. It will be seen that partially charged particles will pick up an additional charge in passing through the centerrnost fibrous material'and will then be collectable in the normal manner. 7 V g It will be'seen that in the specific Cot trell embodiments depicted, the fibrous material 18 may be operativl'ely disposed at a variety of points Within the prec'ipitator. In one application the fibrous -materialinay be preferably concentrated at the input end of the precipitatorto afiord the greatest opportunity for the charged particles; to be electrostatically collected after passing 'therethrough. In a more refined application, however, it 'may be desirable that the flowing gas encounter several fibrous discs disposed at intervals within the precipitator'rather than a single fibrous disc of the equivalent thickness.

,For a particular example, it hasb'eenfound that'by dividing a disc into two discs so asto have the equivalent fibrous material are shown,'it is' understood that it is sectional area of the precipitator by the density measuring means 1'7 in terms of precipitation efiiciency for a typical Cottrell type precipitator, in this instance a 6" diameter precipitator having a 97 /2 inch 12 mil corona wire. For comparison purposes the precipitator was operated first, normally without the fibrous material, as indicated at (a), then second, with A" thick Fiberglas material but no high voltage applied to the corona wire to determine the mechanical filtration effected, as indicated at (b), and'third, with high voltage applied to both the corona wire and the fibrous material, 'as indicated at (c). Another curve indicated at (d) shows the efficie'ncy of the precipitator using both the fibrous material and the high voltagecorona wire, as calc'ula'tedfrom the curves (0) and (b). Considering the'curves (c) and (d), it will be seen that the actual efficiency of applicants device is' considerably better than the theoretical efliciency of the combined means.

While in the'illustrated embodiment several discs of 'within the purview of this invention to use a greater or a lesser number of'fibrous'discs provided, of course, that there'is a 'sufiicient region for the electrostatic collection of charged particles following the last fibrous disc.

"Whereas the device exemplarily' described herein is expressly provided to supplement the charging action of a Cottrell type electrostatic precipitator, it is understood that his within the purview of this disclosure to incorporate'thedevice in other types of electrostatic precipitators for the expressed purpose and that the invention is only to be limited by the scope of the claims appended hereto.

What is claimed is: 1. An improved electrostatic precipitator comprising means for developing an electrostatic field for negatively charging discrete particles 'in a gas flow, means for diirecti'ng'a gas flow containing discrete particles through "said electrostatic field, means for confining the path of said gas flow through said electrostatic field, at least one chargeable fibrous means 'of negative polarity transversely disposed Within said precipitator and in the path of said gas fiow for supplementing the charging action of said electrostatic field, said fibrous means being sufficiently porous to permit passage of said discrete partisaid electrostatic field and to collect particles charged by said fibrous charging means, said collecting means having a polarity opposite to that of said chargeable fibrous "means.

2(An improved electrostatic precipitator comprising -means for developing an'e'lectrostatic'field for negatively charging discrete particles in a gas flow, means for directing a gas flow containing discrete particles through said electrostatic field, means for j'confining the path of said gas flow through said electrostatic field, first and second chargeablefibrous means of negative polarity transversely disposed within said precipitator and in'the pathof s'a'id" gas flow for' s'u'pplementing the charging action of said electrostatic field, said'fibrous means being sufiiciently porous to permit passage of said discrete particles, said gas flow confining means forming means for collecting charged particles in a region along the inner surface thereof and parallel with the direction of the gas flow and along the path thereof within the area of said electrostatic field, said means for collecting charged particles being operable to collect particles charged by said electrostatic field and to collect particles charged by each of said fibrous charging means, said collecting means having :a polarity opposite to that of said chargeable fibrous means.

3. An improved electrostatic precipitator comprising an outer shell conductor having an inlet and an outlet, an electrode of negative polarity extending axially through said outer shell conductor and a voltage source electrically connected thereto, means for directing a flow of gas containing discrete particles of determined average size through said precipitator, a first charging means positioned within said outer shell conductor for charging particles in said gas flow the same charge as said axially extending electrode, a second charging means of fibrous material having the same polarity as said first charging means disposed within said precipitator in the path of said flowing gas for supplementing the charging action of said first charging means, said second charging means being sufiiciently porous to permit the passage of said discrete particles therethrough, said outer shell forming means for collecting charged particles along the inner surface thereof parallel with the path of the gas flow, said means for collecting charged particles being operable to collect particles charged by said first charging means and to collect particles charged by said second charging means, said charged particle collecting means having a polarity opposite to that of said first :and second charging means and said electrode.

4. An improved electrostatic precipitator comprising a hollow column electrode having an inlet and an outlet, a wire electrode of negative polarity centrically disposed along the axis of said hollow column, means for directing a flow of gas containing discrete chargeable particles through said precipitator, a voltage source, means for applying said voltage source to said wire electrode and to said column electrode to develop an electrostatic field therebetween for charging particles in said gas flow, a first chargeable fibrous means of negative polarity transversely disposed within said hollow electrode in the path of said gas flow near the input end thereof for supplementing the charging :action on said particles by said electrostatic field, a second chargeable fibrous means having the same polarity as said first chargeable means transversely disposed within said hollow electrode in the path of said gas flow at a greater distance from said inlet than said first chargeable means to permit subsequent charging of particles in said gas flow, said first and-second fibrous charging means being sufficiently porous to permit the passage of said discrete particles therethrough, said hollow column electrode forming means for collecting charged particles along the inner surface thereof parallel with the path of the gas flow, said means for collecting charged particles being operable to collect particles charged by said first charging fibrous means and to collect particles charged by said second charging fibrous means, said charged particle collecting means having a polarity opposite to that of said first and second charging means and said wire electrode.

5. An improved electrostatic precipitator comprising a hollow column electrode having an inlet and an outlet, a wire electrode of negative polarity centrically disposed along the axis of said hollow column, means for directing a flow of gas containing discrete chargeable particles through said precipitator, a voltage source, means for applying said voltage source to said wire electrode and to said column electrode to develop an electrostatic field therebetween for charging particles in said gas flow, at least one chargeable fibrous means of negative polarity transversely disposed within said hollow electrode in the path of said gas flow for supplementing the charging action on said particles by said electrostatic field, said fibrous charging means being sutficiently porous to permit the passage of said discrete particles therethrough, said hollow column electrode forming means for collecting charged particles along the inner surface thereof parallel with the path of the gas flow, said means for collecting charged particles being operable to collect particles charged by said electrostatic field and to collect particles charged by said chargeable fibrous charging means, said charged particle collecting means having a polarity opposite to that of said centrically disposed wire electrode, said fibrous charging means and said wire electrode.

References Cited in the file of this patent UNITED STATES PATENTS 2,114,682 Gumaer Apr. 19, 1938 2,297,601 Williams Sept. 20, 1942 2,556,982 Roos et a1. June 12, 1951 2,579,445 Warburton Dec. 18, 1951 2,589,463 Warburton Mar. 18, 1952 

